Method of recovering mineral values from an underground formation

ABSTRACT

A HEATED FLUID-PRESSURIZED ZONE IN A SUBSTANTIALLY NONPERMEABLE UNDERGROUND EARTH FORMATION, SUCH AS A CLAYTYPE SHALE, IS SUBSTANTIALLY DYNAMICALLY SELF-CONTAINING AS TO PRESSURE AND IS LARGELY RETENTIVE OF FLUIDS SO LONG AS IT REMAINS AT A TEMPERATURE AT LEAST 10 CENTIGRADE DEGREES, AND PREFERABLY AT LEAST 100 CENTIGRADE DEGREES, ABOVE THE AMBIENT TEMPERATURE OF THE FORMATION. SUCH A SELF-CONTAINED ZONE WHEN SUBSTANTIALLY STATIC IN SIZE IS USEFUL AS A STORAGE PLACE FOR GASEOUS OR LIQUID MATERIALS SUCH AS NATURAL GAS, OR BRINE. SUCH A SELF-CONTAINED, HEATED, FLUIDPRESSURIZED ZONE, MOST GENERALLY WHEN DYNAMICALLY EXPANDING, IS USEFUL IN A PROCESS FOR MINING HYDROCARBON DEPOSITS, E.G., SHALE, OR SOLUBLE MINERAL DEPOSITS, IN A PROCESS WHEREIN THE MINERAL VALUES ARE SEPARATED FROM THE MATRIX MATERIAL BY HEAT AND/OR SOLVENT ACTION WITHIN THE SELFCONTAINED ZONE, AND ARE TAKEN UP BY THE MINING FLUID EMPLOYED FOR PRESSURIZATION AND HEAT TRANSFER, WITH OR WITHOUT CONCURRENT RETORTING ON ACCOUNT OF THE TEMPERATURE OF THE MINING FLUID OR PARTIAL IN SITU COMBUSTION ON ACCOUNT OF OXYGEN SUPPLIED TO CARBON, HYDROCARBON, AND METALLIC SULFIDE MATERIAL IN THE UNDERGROUND FORMATION, AND THEREAFTER MINING FLUID AND MINERAL VALUES ARE RECOVERED FROM THE ZONE BY MEANS OF RECOVERY WELLS, OR POSSIBLY, THE INJECTION WELL ITSELF. THE SELF-CONTAINED HEATED, FLUID-PRESSURIZED ZONE IS CREATED BY INJECTING AT LEAST 20,000 GALLONS OF A MINING FLUID, SUCH AS, WATER, BRINE, OIL OR SULFURIC ACID, INTO A BOREHOLD IN A SUBSTANTIALLY NON-PERMEABLE UNDERGROUND FORMATION AT LEAST 1000 FEET DEEP OVER A PERIOD OF AT LEAST 30 DAYS WITH NO MORE THAN TEMPORARY PUMPING STOPPAGES, DURING WHICH THE PRESSURE AGAINST THE FORMATION IS SUBSTANTIALLY NOT RELIEVED, AND THE MINING FLUID AT THE WELLHEAD HAS A TEMPERATURE AT LEAST 100 CENTIGRADE DEGREES ABOVE THE TEMPERATURE OF THE UNDERGROUND FORMATION. TYPICALLY, PUMPING OF A MUCH LARGER VOLUME IS CARRIED OUT OVER A ONE YEAR PERIOD OR MORE. THE UNDERGROUND FORMATION CRITICALLY HAS AN INHERENT PERMEABILITY NOT SUBSTANTIALLY GREATER THAN A CLAYTYPE SHALE SUCH AS THE ANTRIM SHALE.

Sept. 20, 1971' A. w. HANSON 3,606,465

METHOD OF RECOVERING MINERAL VALVES FROM AN UNDERGOUND FORMATION viled March 12, 1.969 6 Sheets-Sheet 1 INVENTOR.

A /aen W. Hanson fiTTOR/VE Y A. W. HANSON METHOD OF RECOVERING MINERAL VALVES FROM AN UNDERGOUND FORMATION Sept. 20, 1971 6 Sheets-Sheet 3 Filed March 12. 1969 HTTORNfY Sept. 20, 1971 A. w. HANSON METHOD OF RECOVERING MINERAL VALVES FROM AN UNDERGOUND FORMATION 6 Sheets-Sheet 5 viied llarcn 12. 1969 TTORNL'Y Sept. 20, 1 971 A. w; HANSON METHOD OF RECOVERING MINERAL VALVES FROM AN UNDERGOUND FORMATION I 6 Sheets-Sheet 4 Filed larch 12, 1969 2 65.3 (m QQU o b .C QUESU x wbs uw k g fiux Q x MRMW 6 47 70; wza

INVENTOR. filo en M/. Hanson p 20, 1971 w. HANSON METHOD OF RECOVERING MINERAL VALVES FROM AN UNDERGOUND FORMATION 6 Sheets-Sheet 5 Filed larch 1.969

INVENTOR. A/oen W. Hanson HTFORNEY HANSON Sepf. 20 1971 A METHOD OF RECOVERING MINERAL VALVES FROM AN UNDERGOUND FORMATION 6 'Sheets-Sheet 6 Filed larch 12, 1969 INVENTOR. HF/den W. Hanson HTTORNfY United States Patent 3,606,465 METHOD OF RECOVERING MINERAL VALUES FROM AN UNDERGROUND FORMATION Alden W. Hanson, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich. Filed Mar. 12, 1969, Ser. No. 806,606 Int. Cl. E21b 43/24 U.S. Cl. 299-4 19 Claims ABSTRACT OF THE DISCLOSURE A heated fluid-pressurized zone in a substantially nonpermeable underground earth formation, such as a claytype shale, is substantially dynamically self-containing as to pressure and is largely retentive of fluids so long as it remains at a temperature at least Centigrade degrees, and preferably at least 100 centigrade degrees, above the ambient temperature of the formation. Such a self-contained zone when substantially static in size is useful as a storage place for gaseous or liquid materials such as natural gas, or brine. Such a self-contained, heated, fluidpressurized zone, most generally when dynamically expanding, is useful in a process for mining hydrocarbon deposits, e.g., shale, or soluble mineral deposits, in a process wherein the mineral values are separated from the matrix material by heat and/or solvent action within the selfcontained zone, and are taken up by the mining fluid employed for pressurization and heat transfer, with or without concurrent retorting on account of the temperature of the mining fluid or partial in situ combustion on account of oxygen supplied to carbon, hydrocarbon, and metallic sulfide material in the underground formation, and thereafter mining fluid and mineral values are re: covered from the zone by means of recovery wells, or possibly, the injection well itself. The self-contained heated, fluid-pressurized zone is created by injecting at least 20,000 gallons of a mining fluid, such as, water, brine, oil or sulfuric acid, into a borehole in a substantially non-permeable underground formation at least 1000 feet deep over a period of at least 30 days with no more than temporary pumping stoppages, during which the pressure against the formation is substantially not relieved, and the mining fluid at the wellhead has a temperature at least 100 centigrade degrees above the temperature of the underground formation. Typically, pumping of a much larger volume is carried out over a one year period or more. The underground formation critically has an inherent permeability not substantially greater than a claytype shale such as the Antrim shale.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention relates to a self-contained highly pressurized zone in an underground earth formation of a nonpermeable nature, such as a clay-type shale; to the method of creating such a self-contained zone by the injection of a heated mining fluid over a substantial period of time; and to a method of mining or recovering mineral values, including hydrocarbons, sulfur or soluble salts, from a non-permeable underground earth formation utilizing such a self-contained zone in a dynamically expanding condition.

For the purposes of the specification and claims normally impermeable means substantially non-permeable to a fluid applied at a pressure inherent to the formation in situ. A non-permeable formation is defined as any formation having a permeability less than 0.1 millidarcy to compressed air and a grain size less than about 0.0625 millimeter. Such a formation is subject to fracturing with little collateral rupturing. A borehole in such a formation substantially does not yield water nor does the surrounding formation take up water even when the borehole is at least percent filled with water.

A typical shale formation advantageously treated according to the present method exhibits an average grain diameter of about 0.004 millimeter.

The term communicative fracturing, sometimes referred to as substantially single path type fracturing, refers to fracturing of a formation in a manner such that the fracturing liquid or fluid follows either a single path or a small number of paths outwardly from the point of injection and often extending to a recovery well or other means of escape from the formation. The path will not ordinarily follow a straight line but will be directed from the inlet to the point of greatest extension or to the recovery well substantially without side branching. Communicative fracturing is to be contrasted to intimately branched or pulverizing collateral fracturing in which a very substantial amount of side branch fracturing occurs creating a great multiplicity of pathways outwardly from the point of injection. Communicative fracturing always occurs at least initially if pressure against the formation is raised sufliciently. The intimately branched, collateral fracturing occurs in following the present method and is desirable.

The term formation pressure refers to the pressure the formation being treated underground is able to withstand substantially without accepting pressurizing mining fluid. As a minimum, this pressure is ordinarily directly related to the weight of the overburden, but is increased substantially on following the method of the invention.

The term mining fluid refers to any liquid, vapor or fixed gas advantageously injected into a formation according to the invention to act as a heat transfer agent, solvent, and/or reactant, in effecting release, or separation, and recovery of mineral values from the matrix material.

The overburden lifting pressure is the normal fracturing pressure of a formation. In sedimentary formations this pressure falls, typically, at a numerical value in the range of about 0.4 to about 1.1 pounds per square inch per foot of depth, when using an injection fluid substantially at the ambient temperature of the formation. In general, fracturing of a sedimentary formation at ambient temperatures occurs when the stress exerted by the weight of the overlying overburden is overcome, although other factors such as the elastic properties and strength of the rock, and faults in and arching of the formation also contribute to the exact formation pressure encountered.

(2) Description of the prior art Heretofore it has been found, on injecting a fluid into an underground formation at substantially the ambient temperature of the formation, that the formation fractures and accepts the fluid at the computable overburden lifting pressure. In general, the formations into which hot fluids have been injected have been quite porous materials such as sand or sandstone rock and the like. On the occasions when the formation has been a substantailly nonpermeable formation,.injection has been limited to relatively small volumes of fluid injected for short time periods and at relatively low pressures, all insufficient to heat a substantial amount of the formation, or if a zone in the formation was heated, sufliciently high pressure has not been employed to substantially fracture the formation and enlarge such a heated zone.

OBI ECT S OF THE INVENTION In presurizing underground formations, as in hydro carbon recovery from a shale formation containing hydrocarbons, various problems have arisen on attempting to control penetration of the earth formation. In order to inject a hot fluid mainly in the liquid form in such a way as to avoid vaporization of any liquid portions, it is essential that the formation pressure be raised. Vaporization of the injected fiuid results in the temporary loss of heat energy at a high temperature and pressure. Recovery of the heat energy then occurs at a lower, less useful temperature when the vapors condense at a lower temperature further out in the formation. This is an important factor to consider, especially in the case of water, because of its high heat of vaporization. The more desirable goal, generally, is to bring about heat transfer to the formation at a higher, more useful temperature.

In retorting or igniting hydrocarbon in shale, it becomes criticalto bring the formation to the temperature at which decomposition of the kerogen takes place along with distillation, thermal cracking, or ignition, as desired.

It is also highly desirable in distillation of a hydrocarbon-containing shale formattion, to retort as much of the bed as possible between the injection well and an adjacent prgduction well or wells. If the formation fractures in a manner to provide too ready communication between the injection well and the production well, and if the communicating fractures are not controlled, there is a channelling effect and only a small part of the formation immediately adjacent each communicating fracture becomes heated sufliciently to permit processes such as retorting or igniting of the hydrocarbons. What is desirable is suflicient containment of the injected material by the formation whereby pressure against the formation builds up for multiple, intimately branched fracturing of the formation to take place throughout the surface of a burgeoning, ever increasing volume of formation, and creation of a substantially continuous front along such surface where processes such as hydrocarbon recovery, as by retorting, or mineral recovery, as by leaching, take place. It is also highly desirable to bring about, at surfaces produced or exposed by fracturing, penetration into the rock even without further fracturing thereof.

Uncontrolled communicative or substantially single path type fracturing, rather than the controlled fracturing which will result in intimately branched or pulverizing collateral fracturing, is likely to occur in any underground earth formation, such as Antrim shale, where permeability is low. Any formation having a permeability less than 0.1 millidarc'y to compressed air and a grain size less than about 0.0625 millimeter is subject to fracturing with little collateral rupturing unless controlled according to the invention. Channelling is not completely avoided, but continues ahead of the heated zone and provides the main production channels, eventually, to the output well or wells. It is the detrimental effects of channelling, and not desirabel effects, that need to be overcome according to the invention.

It is, therefore, a princial object of the invention to provide a method of controllably raising the pressure in a relatively non-permeable underground earth formation and thereby to create a self-contained, high-pressure zone within the formation.

Another object of the invention is to provide a method of creating a heated, self-contained, dynamic, expanding, pressurized zone in a non-permeable underground earth formation while avoiding uncontrolled channelling type communicative fracturing of the formation.

Another object of the invention is to provide a method of creating a heated, self-contained, dynamic, expanding, high-pressure zone in a non-permeable underground earth formation having a sufliciently high temperature within the self-contained zone to provide for one or more preselected processes such as retorting, controllable burning, or leaching, thereby to recover mineral values from the formation.

Another object of the invention is to provide a selfcontained, dynamic, high pressure zone in a non-permeable underground earth formation that is useful as a reservoir.

Another object of the invention is to provide a method of intimately penetrating a normally non-permeable underground formation by permeation rather than solely by fracturing.

Another object of the invention is to provide a method of controlling the penetration of fluid into a previously fractured formation.

Still another object of the invention is to provide a method of treating an unnderground earth formation in place to recover mineral values therefrom.

Yet a further object of the invention is to provide a method of maintaining higher pressures and temperatures in fluids injected into a non-permeable underground earth formation by means of greater injection temperatures.

These and other objects and advantages of the present invention will be more clearly understood by those skilled in the art upon becoming familiar with the following description.

SUMMARY OF THE INVENTION It has now been discovered that upon injecting at least 20,000 gallons of mining fluid into a non-permeable type underground earth formation, at least 1000 feet deep, in a substatnially continuous manner and over a period of at least about 30 days with no more than temporary stoppages in pumping, the fluid fracturing material at the wellhead being at a temperature at least centigrade degrees above ambient formation temperature, the injection pressure exerted against the formation and susstainable by the formation may be brought to a level substantially above the normal overburden lifting pressure, and, throughout a useful amount of earth formation, a heated, dynamic, self-contained high pressure zone largely retentive of fluids is created which is capable of expanding dynamically and controllably upon continued injection. Preferably the temperature is brought sufficiently high that a formation pressure at least 10 percent greater than normal fracturing pressure is obtained in a dynamic manner in which highly branched, intimate fracturing takes place, as injection proceeds. Preferably the temperature of the injected mining fluid is sufliciently high to raise the formation pressure at least 100 percent above normal fracturing pressure. In mining operations carried out to recover mineral values from a formation, in place, it is often advantageous to carry out injection of mining fluid over a period of a year, or more.

DETAILED DESCRIPTION OF THE INVENTION The pressure required to fracture an underground earth formation is found, almost universally, to be in the order of 0.4 to 0.8 pound per square inch (p.s.i.) per foot of depth of the formation, except for an occasional unusual formation where the pressure has been known to go to 1 to 1.1 p.s.i. per foot of depth. Upon subjecting a formation to such fracturing pressures or greater pressures by the injection of a liquid at or near ambient formation temperature, the formation is found to fracture when the overburden lifting pressure is reached. Such pressure is herein referred to as the normal fracturing pressure.

Normal fracturing, if uncontrolled, takes place in an unwanted manner as far as the purposes of retorting or mineral leaching are concerned, though some normal fracturing is needed, usually, to effect recovery from the treated zone. Normally, single path or communicative fracturing proceeds or propagates in such a manner as to by-pass large masses of rock containing the values to be recovered, while the cracks obtained on such fracturing are so far ranging that formation pressure is hard to maintain at a high level because of lack of fluid retention and rapid movement of fluid to an output well may occur before the fluid is enriched with mineral values.

By proceeding according to the present invention, it has been found possible to control in a general way, the propagation of fractures and to raise dynamic formation pressures to levels of the order of about 1.5 to 2 p.s.i. or more per foot of depth. Such pressure increase occurs while there is relatively little loss of the injected fluid from the desired intimate contact zone into the advance cooler zone since channelling type communicative fracturing is controlled and reduced to low but essential levels. It is even possible to control the penetration of injected fluid into a previously fractured formation that, but for the fractures, would be a substantially non-permeable formation.

The mechanism is not entirely understood, but it is believed that the behavior of a formation subjected to the present method of treatment is explainable on the basis of thermal expansion of the treated formation upon its being heated by the hot fluid being injected. It appears that expansion of the rock results in the taking up of void spaces with a consequent closing and tightening of the formation giving it greater horizontal rigidity so that up-lifting pressure exerted at any given point tends to raise and support a substantially larger conical portion of the overburden than would normally be expected from experience in using conventional fracturing techniques. That there is greater mass being lifted is believed to be reflected in greater dynamic formation pressure resulting in greater containment of the injected liquid within a given zone surrounding the point of injection. As a consequence of the great attainable pressure, substantially enhanced permeation occurs whereby good fluid penetration and passage is achieved within the zone of containment, in addition to intimate fracturing along the boundaries of the zone. Rock fragments or masses, resulting from fracturing operations, when treated according to the method of the present invention, are further subjected to penetration or permeation by fluid moving into the normally impermeable unsubdivided fragments. Such penetrating permeation provides for product recovery from permeation effects alone. It is to be understood that such permeation effects are supplemental and complementary to product transfer as a result of fracturing alone. Permeation at the pressures attainable by the present method is highly significant since experience has shown that a substantial fraction, for example, about half the fluid accepted by the formation at elevated pressures above normal fracturing pressure permeates the rock and may not be recovered on relieving the pressure, except by displacement. In addition, and highly significantly, there is obtained not just a single fracture, nor even a small number of fractures, which carry fluid flow in any given direction in a highly preferential manner, but any fracture opened up is soon closed by the thermal effect whereupon at any given constant rate of pumping the applied pressure rises a little and opens up a new fracture which shortly closes, and this process is repeated over and over in a band or shell at the outer boundary of the dynamically contained zone while communicative fracturing gradually proceeds ahead of this zone.

Generally, a temperature increase over ambient formation temperature will cause at least some expansion of the rock and a closing of void spaces in non-permeable formations. Generally sustained injection of a fluid, having, at the well-head, a temperature 100 centigrade degrees above ambient formation temperature, over a period of 30 days or more and in a total amount ofat least 20,000 gallons, is suflicient to cause a noticeable increase in formation pressure throughout a useful amount of the formation. Useful temperatures of injection are those causing the dynamic formation pressure to rise at least percent after 30 days of operation. The formation may usefully be heated to any desired temperature above that causing such 10 percent pressure rise. The upper temperature limit is generally dictated by the nature of the operation it is desired to carry out. In some instances it will be desirable to operate at temperatures in the range of 80 C. to the critical temperature for water (373 C.), as in the leaching of mineral values with water or with an aqueous leachant solution. In other instances it will be desirable not to exceed the temperature at which the hydrocarbon content of the shale formation retorts readily, e.g., 275 to 395 C. In other instances it will be desirable to reach temperatures at which material in the formation ignites and controlled combustion is maintained on injecting air or oxygen. In instances where there are no process limitations on the upper temperature limit,

it will be necessary to avoid temperatures at which the formation itself decomposes or breaks down whereby pressure control is lost. In most instances, it is desirable to avoid the taking into solution of large quantities of silica, where, for example, aqueous liquid is injected into a siliceous formation. Otherwise, the upper temperature limit often is dictated by pressure containment limitations in the equipment used to inject hot fluid into the formation.

As a general rule, in treating a hydrocarbon-containing shale formation to recover the hydrocarbons, it is desirable to heat the formation by the injection of a fluid having a temperature of at least C. and much more preferably, a temperature of at least 250 to 500 C. To provide for maintenance of higher temperatures in the injected fluid, especially where the fluid is water, and to facilitate intimate fracturing and thorough permeation within the contained zone, it is best to employ a temperature sufiicient to create a dynamic injection pressure of at least 2,000 p.s.i. at the face of the formation, and more preferably, for most work, a pressure of 3,000 to 6,000 p.s.i. Such pressures are not attainable at depths less than about 1,000 to 1,500 feet without fluid loss to the ground level.

The formation may be heated in any desired manner, and most any heating means may be placed in the formation, generally that which is dictated by economics. Usually heating is carried out by injection of the mining fluid used for pressurizing the formation since intimate contact of nearly all parts of the rock with the heat source is required. The mining fluid may be heated above ground in a conventional boiler utilizing either fossil fuel or nuclear fuel, or underground using a conventional fuel or nuclear fuel heated boiler, or using electrical heaters if economically feasible.

Heating the formation in any of these ways may also be supplemented by carrying out controlled combustion of e.g., hydrocarbon, carbon or metallic sulfide material within the self-contained zone. Such combustion process is attractive when the formation is one containing hydrocarbon material, such as Antrim shale, whereupon it is sufficient to ignite a mixture of retorted hydrocarbons and air or oxygen, injected alone as, or along with, the regular mining fluid. Carbon, mainly, remains in the rock after all volatile hydrocarbons are removed and is the fuel of choice. The combustion process itself is initiated, proceeds and is controlled generally in a manner similar to known underground combustion process%, but within the context of the present self-contained zone processes.

The combustion process is also attractive when the formation contains metallic sulfide and sulfur values are recoverable therefrom in a commercially attractive amount in the form of a volatile gas, such as S0 Generally creation of a temperature, within the formation, of about 550 to 700 degrees centigrade is essential to oxidation of the metallic sulfides in situ at practically useful rates.

Most any liquid with a relatively high vaporization temperature will do as a mining fluid. Suitable mining fluids include water, brine, aqueous mineral acid such as sulfuric acid or hydrochloric acid, oil, such as mineral oil, ethylene glycol, propylene glycol, or any liquid which does not too rapidly corrode the heating, pumping and well equipment. It is to be further understood that fluids, as used herein, includes not only the fixed gases, such as oxygen or air, but also the vapor forms of such liquids as water and mineral oils.

While the pressure in the formation begins increasing promptly upon injecting a hot fluid into the formation,

the heating of the formation proceeds relatively slowly and high formation pressures in the order of at least 2,500 to 3,000 p.s.i. are not attained upon injecting a small volume of fluid at a modest temperature and over a short period of time. Generally a period of two or more weeks are required to raise the pressure of a newly penetrated non-permeable formation located at a depth of about 1,500 feet to a pressure of the order of 4,000 to 5,000 p.s.i. Thus, it would be expected that relatively little useful increase in formation pressure would be observed upon injecting less than about 20,000 gallons of heated fluid unless the temperature is unusually high. More generally, at least 120,000 gallons of warm to hot fluid heat transfer material must be injected at a steady, substantially uninterrupted pace to substantially and usefully increase formation pressure. As a practical matter, the process of injection is best carried out over a period of at least six months and more preferably a year or more. Injection for periods of two to three years or even longer would constitute a desirable major investment which would thereafter pay off for a number of years.

Recovery of fluid from pressurized formations is a relatively slow continuous process of relieving the pressure, as through a recovery well, since fluid forced into the nonpermeable formation must bleed back out through natural pores where permeation has taken place. Recovery generally proceeds for a useful period of at least two to three or more times the injection period. Thus, a formation pressurized over a period of one year may be expected to yield useful quantities of fluid for at least two to three years, and generally, for an even longer time.

By way of illustration, it has been found that each centigrade degree of temperature rise, in an Antrim shale formation at a depth of about 3,000 feet and at a temperature of about 300 C., reversibly increases the sustainable formation pressure about 9 p.s.i.

A self-contained, dynamic, pressurized zone created according to the present invention in a substantially nonpermeable formation at least 1,000 feet deep, having a volume of at least 50,000 cubic feet and preferably 500,000 cubic feet, and a temperature 100 centigrade degrees, and perferably 150 centigrade degrees, above the surrounding formation, is highly useful as an economically created temporary storage vessel for fluids such as natural gas or brine or chemical waste.

Operations according to the invention will be better understood on becoming familiar with the following description and the appended drawings in which:

FIG. 1 shows a schematic arrangement of boreholes or wells that are to be understood to be penetrating into an underground earth formation, the arrangement consisting of a single input well surrounded by an array of recovery wells;

FIG. 2 shows a schematic arrangement of wells representing a subsequent stage after the substantially complete development of the field of FIG. 1, the recovery wells of FIG. 1, in effect, having been converted to input wells and surrounded by a concentric ring or array of recovery wells;

FIGS. 3 and 4 each represents successive stages in an embodiment of a mode of development differing from that shown in FIG. 2, in which rings of recovery wells overlap the original array of FIG. 1, successive rings or arrays being deployed successively until the field concentrically around the array of FIG. 1 has been gradually worked over;

FIG. 5 is a small fragmentary view, actual size, of a vertical section through an underground formation intimately fractured by the dynamic expansion therethrough of a self-contained high-pressure zone according to the invention;

FIG. 6 is a schematic representation of a momentary profile taken through an underground formation between an input well and a recovery well, indicating various processes and changes in physical conditions occurring at various locations between the wells according to the process of the invention;

FIG. 7 is a schematic representation similar to that shown in FIG. 6 but showing a momentary profile in an operation in which combustion is being carried out in the self-contained zone according to the process of the invention;

FIG. 8 is a schematic representation showing changes with distance, from the input well, in the various factors that add up to give the net permeability of the formation as a consequence of the processes occurring according to FIG. 6;

FIG. 9 is a schematic respresentation similar to FIG. 8 showing the various factors affecting the net permeability of the formation as a consequence of processes occurring according to FIG. 7; and

FIG. 10 is a schematic representation similar to FIGS. 7 and 8 showing the changes in net permeability upon attempting to carry out the processes of the operation shown in FIG. 6, but in a permeable formation, the process being therefore outside the scope of the invention.

It is to be understood that the relative sizes of zones indicated in each of FIGS. 6-10 are not necessarily indicated by the relative dimensions depicted in the schematic representations.

In carrying out mining operations according to the process of the invention, as for example, in the recovery of hydrocarbon material from an underground clay-type shale formation at least one thousand feet deep and preferably at least two thousand feet deep, an injection well is drilled into the formation, for example 1 in FIG. 1 of the drawings. An array of recovery wells preferably are provided spaced apart from the injection Well from about mile to /2 mile or more, depending on the extensiveness or thickness of the field or vein of shale, or other mineral deposit, as well as upon the size of investment intended and the capabilities of the heating and pumping equipment employed.

For example, an array of recovery wells spaced about a circle concentric to 1 the individual wells being designated by R are shown in a suitable pattern in FIG. 1. In general, the further the spacing between the injection and recovery wells, the greater is the period of time and the volume of mining fluid required for the self-contained zone to advance to each recovery well, but further, the greater is the period of time during which production is normally reaped from the recovery well or wells.

After pressurizing and heating the formation and recovering substantially all the mining fluid that can be taken up from the recovery well or wells at any economically reasonable production rate, and so long as the underground formation being mined in place is sufficiently extensive, it is generally desirable to close in the original injection well and to provide a new concentric array of recovery wells, while using one or more of the original recovery wells as injection or input wells, for example, in an array as shown in FIG. 2..

In another manner of proceeding after exhausting the recoverable mining fluid from the initial field, the zone being mined may be enlarged more slowly and gradually 1n successive steps as depicted in FIGS. 3 and 4. In FIG. 3, the original input Well has been changed into a recovery well, one of the recovery wells has been converted into an injection well, I and three new recovery wells, plus two of the original recovery wells and the converted input well, are disposed in concentric circular array around the recovery well converted to an input well, I In FIG. 4, such an approach has been taken one step further. The original input well and substantially all the recovery wells but one are closed. The second input well I is closed or converted to a recovery well as R One of the second stage recovery wells is converted to an input well, 1 three new recovery wells are provided and two of the recovery wells from the second stage shown in FIG. 3 continue to be employed so that the input well, I is surrounded by a concentric circular array of recovery wells, R In a similar manner, additional stages are followed round and round in a substantially spiral pattern until the field is substantially worked over or exhausted.

The processes which take place and the phenomena that make such processes possible will be better understood with reference to FIGS. 6-10 in which there are shown momentary profiles of temperature, fluid pressure, and gross permeability of the underground formation at the level being treated according to the process of the invention taking place between the input well and the output or recovery well. Referring now to FIG. 6, there is shown a schematic representation of the temperature, fluid pressure, and gross permeability conditions on a bilinear graph in which numerical values increase along the vertical scale and the distance of any given phenomenon from the input to the recovery well is the variable along the horizontal scale. The profile shows conditions prevailing, after the self-contained zone has been moved about half way between the input well and the recovery well.

Fluid flow starts at the input well and moves substantially radially outward therefrom. In the plane represented by the profile, the mining fluid moves to the left toward the recovery well represented by the left side of the graph. All the processes shown taking place between the wells have already occurred adjacent the input well, and the various zones have progressed through the formation until spread out at the moment represented by the profile. Mining fluid moving out from the input well is seen to cool gradually while it traverses what is now a moderately permeable formation at a very substantial fluid pressure.

As the mining fluid moves through the formation and gradually cools, it is found that there is a normal pressure drop, and also, the formation further from the Well is somewhat less permeable because there has been a shorter contact period with the mining fluid and less opportunity for retorting, or leaching, or other separation and recovery processes to progress to completion and increase the porosity of the formation by removal of mineral values.

About halfway between the wells at the moment depicted, retorting of mineral values such as hydrocarbons is taking place in and right behind a zone of dynamic thermal seal that advances toward the recovery well as injection of mining fluid proceeds.

The zone of thermal seal is a peripheral, enveloping shield or blanket that is dynamically created, maintained and/or moved, and is an artifically created barrier to fluid flow that enlarges and balloons outwardly from the input well as injection of heated mining fluid is continued. Because the barrier shield or blanket is a dynamic thing, especially during expansion, when fractures open a little way, warm fluid flows briefly closing such fractures whereupon new fractures open, and so on. The barrier shield or blanket is not very sharply defined but extends through an enveloping layer of underground formation having a thickness of about 10 to 50 feet or more and is, perhaps, for this reason, more properly described as a Zone than as a shield or blanket.

On the graph of FIG. 6, the limits of the zone of thermal seal are indicated by points A and B where the curve for gross permeability crosses the line for normal permeability of the substantially non-permeable formation. Intermediate the points A and B, the gross permeability very nearly reaches zero. Within the zone, the temperature of the formation is sharply lower with distance from the region already heated. Warm fluid moving into the advancing front penetrates any given fracture haltingly, cools while warming the surrounding rock, and stops while other fractures open. Gross permeability is less in the middle part of the zone because little freeing or separating of mineral values has taken place and the thermal expansion seems to have closed the rock. Towards the advance side of the zone where the mining fluid has penetrated the least, the temperature of the formation is moderately low, the permeability is actually greater than in the middle of the zone because there is less thermal valve effect, and fluid pressure is moderate because of substantial pressure drop through the sealing zone.

Processes, such as retorting, that are dictated by temperature conditions, initiate once temperature is sufliciently great, for example, as here, after about half of the zone of thermal seal has passed any given portion of the formation. Such processes as retorting continue so long as material remains to be retorted, after which, and more generally, partly concurrently therewith, leaching and other processes also take place. The products of such mining processes become a part of the mining fluid, and the whole melange is pushed toward the production well as injection proceeds.

vOut in front of the zone of thermal seal, i.e., to the left of such zone, as shown in FIG. 6, normal fracturing is occurring, being caused and propagated by the movement of cooler mining fluid which has been initially injected at the input well and pushed ahead of the zone of thermal seal, has been cooled by traversing a substantial amount of formation, and is continuously added to and pressurized by mining fluid escaping from the dynamic thermal seal. Since the fluid is cooled, there is no thermal valve effect to prevent fracturing at normal overburden lifting pressures, and also, mining processes occur slowly or not at all in the presence of the cooler mining fluid.

Examining the formation ahead of the zone of thermal seal and up to the points of greatest penetration by the mining fluid, it is found that the temperature of the formation is cooler with distance away from the heated zones, the pressure drop reflects the traversal of fractured, normally non-permeable rock up to the point of greatest penetration where inherent pressures of an untouched formation obtain, and the gross permeability, after increasing with decreasing thermal effect, decreases with distance, due to decreased fracturing having occurred, until normal permeability obtains where the formation is untouched.

It is to be understood that the process of the invention, at the moment depicted in FIG. 6, has not proceeded to the point that recovery of mineral values can be carried out at the recovery well, though some recovery could be made back through the input well, if desired. Injection must proceed, generally, until at least normal fracturing occurs around the recovery well. Ordinarily, injection will proceed until the retorting zone has reached the recovery well, when mining fluid containing mineral values may be drawn out of the recovery well, or the recovery well may be turned into an input well and the mining fluid may be pushed through an even larger zone to a newer, more distant recovery well or wells.

Referring now to FIG. 8 together with FIG. 6, the individual factors which sum up to gross permeability and which change according to the process occurring in FIG. 6, are depicted in momentary profile curves. The actual gross permeability representing the sum of the factors is shown as a profile curve along with the curve for (1) hypothetical gross permeability in which the effect of thermal expansion has been omitted or subtracted, (2) permeability due solely to fractures, (3) permeability due solely to porosity in rock matrix (4) permeability due solely to thermal expansion, shown against the background of (5) permeability of the unaltered formation. In the curve reflecting the sum of all these factors, viz, actual gross permeability, the points marking the intersection of such curve with the fixed curve for permeability of unaltered formation are labeled, respectively, A and B. Across this portion of the formation, i.e., between A and B, lies the zone known as the dynamic thermal seal.

Injected fluid does not cross the barrier represented by such seal except by random repetitious scattered fracturing, each such fracture being promptly closed again by the thermal expansion effect. The crossing of the actual gross permeability curve at point B occurs because of the substantially increased porosity of the rock matrix as a consequence of the separation of mineral values as the mining process proceeds, the mineral values being taken up by or released by the heated mining fluid, for example, in the process indicated in FIG. 6, retorting, as of hydrocarbon shale, is taking place in and adjacent the zone of dynamic thermal seal.

Substantially similar processes are shown schematically as taking place between the input and recovery wells in the representation of FIG. 7, except that the mining fluid is being injected at a sufiiciently high temperature and in admixture with or intermittently with air or oxygen or other highly reactive gas so that controlled combustion takes place adjacent the retorting zone. One of the major differences, between the processes of FIGS. 6 and 7, is that the temperature in the combustion zone is higher than the temperature at the input well. In retorting processes without combustion the greatest temperature level is found at the input well since there is a cumulative temperature loss occurring with distance and time.

Referring to FIG. 9, the individual factors which sum up to the gross permeability shown in profile in FIG. 7 are depicted in a manner similar to that of FIG. 8. Again the points where the actual gross permeability curve dip below and cross the fixed line for permeability of the unaltered formation are marked respectively A and B. Points A and B coincide with the vertical lines marking the limits of the zone of dynamic thermal seal.

By way of comparison of processes of the invention taking place in a substantially non-permeable formation, as contrasted to treating a permeable formation in the similar manner, there is shown, in FIG. 10, the momentary profiles, between an input Well and a recovery well of individual factors which sum up to gross permeability in such a permeable formation. While changes in permeability due solely to thermal expansion would tend to reduce total permeability rather drastically, the rapid passage of heated mining fluid through the permeable formation raises the temperature of the formation quite rapidly and retorting or leaching start taking place promptly, resulting in sufficiently rapid taking up of mineral values that the formation exhibits a large increase in porosity and fracturing also occurs readily. The result is that thermal expansion is never able to close the formation, as seen by the fact that the actual gross permeability curve lies well above even the borderline between the permeability levels of non-permeable and permeable formations. The net result is that no self-contained zone whatever is formed in such a permeable formation and thus treatment of such a formation constitutes a process outside the scope of the present invention.

EXAMPLES The following examples serve to illustrate the method of the invention and the invention is not to be considered limited thereto.

Example 1.-Water injection equipment was set in place about a borehole extending into an Antrim shale formation. The formation is about 400 feet thick in the vertical direction, is extremely extensive in the horizontal directions, and lies at a depth of about 3,000 feet. On the morning of the first day of the test, injection of hot water was commenced at a wellhead pressure of about 1,625 pounds per square inch gauge (p.s.i.g.). By noon the injection temperature had increased to 290 C. and a well head pressure of about 2,050 p.s.i.g. was being applied, an injection rate of about 40 gallons per minute of hot water being maintained with few interruptions. Injection under these conditons was maintained for 4 days, when the injection temperature was gradually raised to 350 C. Injection temperatures referred to herein were measured at the boiler outlet.

The following day the injection temperature was raised 12 to 375 C. for a short period, then dropped back again to 350 C. Injection continued under these conditions for four days when the injection temperature was increased to 385 C. and maintained at that temperature for eight days.

The injection pressure necessary to make the hot water enter the formation at a steady rate increased steadily throughout the test as the formation heated up around the borehole and within the zone of containment. Pressure measurements, taken and recorded during the injection period, indicate a wellhead pressure of 3,800 p.s.i.g. at an injection temperature of 385 C. The recorder charts indicate that the pressure was continuing to rise and it is estimated that the maximum inlet pressure might go as high as 4,500 p.s.i.g. at the 26.8 gallon per minute flow rate used at the terminal part of the treatment. Adding 600 to 700 p.s.i.g. for the hydrostatic head of hot water results in an estimated 5,100 to 5,200 p.s.i.g. pressure at the face of the formation.

At the conclusion of the foregoing period of hot water injection, the temperature rise at two output Wells located, respectively, 900 feet and 1,378 feet from the input Well was negligible, i.e., less than 2 degrees centigrade.

The normal fracturing, or overburden-lifting, pressure of the formation tested is about 2,050 pounds per square inch gauge on using an injection fluid at the ambient temperature of the formation.

In a cold water injection test made by way of comparison to the foregoing hot Water test, Water having a temperature of about 15 to 25 degrees centigrade was pumped into the same borehole described above. Injection of such water continued at a rate of about 28 gallons per minute for a number of days. Then 1,000 gallons of an aqueous bromide solution containing 2,000 pounds of sodium bromide tracer was injected into the borehole in about 1 hour and fifteen minutes. After that, cold water injection proceeded as before.

Approximately 20 hours after the injection of the sodiumbromide solution into the borehole, bromide-containing water was produced at the well head of the outlet well 1,378 feet away. At a second outlet well, located 900 feet from the inlet borehole and in a different radial direction, bromide was detected at the Wellhead 32 hours after injection.

The foregoing test shows the rapid formation of communicative pathways between input and output wells upon injecting fluid into an earth formation at about ambient temperatures.

Example 2.An injection test was carried out at the same input borehole used in the test described in Example 1. Injection of water having a temperature of 60 degrees centigrade was commenced at 1,500-11, 600 p.s.i.g. and at a rate of 15,000 to 17,000 pounds per hour. The injection water temperature was raised to over 200 degrees centigrade and injection continued for about 6 hours. Then the well was closed overnight and injection was started up again the next morning. On the second day, hot water at about 300 degrees centigrade was supplied for injection and injection continued for about 7 hours. During this time, automatic controls set to limit the dynamic injection pressure to 2,500 to 2,700 p.s.i.g. throttled back the injection rate to 2,700 to 3,000 pounds of hot water per hour, showing that the formation was containing and dynamically holding the hot Water.

To show the reversibility of the behavior of the formation, the hot water injection was stopped and water at 60 degrees centigrade Was injected into the same borehole. Within about 2 hours, the injection pressure fell to 1,500 to 1,600 p.s.i.g. and the input rate roseto about 15,000 to 17,000 pounds per hour.

I claim:

1. The method of creating a heated, self-contained, dynamic, expanding, high-pressure zone in an underground earth formation containing mineral valuesand having an average grain size less than about 0.0625 mm. diameter and an average permeability less than 0.1 millidarcy to air, said underground formation being at least 1000 feet deep and penetrated by a borehole, which comprises:

over a period of at least 30 days, pumping down the borehole and injecting into the formation a total of at least 20,000 gallons of heated mining fluid with no more than temporary stoppages in pumping and while substantially maintaining the pressure against the formation whenever pumping is temporarily stopped. said heated mining fluid having, at the wellhead, a tempermature at least 100 centigrade degrees above the ambient temperature of the underground earth formation,

suflicient mining fluid being injected to exceed normal fracturing pressure for that formation,

the heated mining fluid in the earth formation being at a sufliciently elevated temperature that the expanding high-pressure zone remains substantially self-con tained,

the high-pressure zone created being characterized as self-containing of pressure therein for temporary periods even when pumping is stopped and dynamic expansion is substantially stopped so long as pressure against the formation attained by pumping is substantially not allowed to fall,

removing mineral values from said underground earth formation,

and recovering mining fluid and said mineral values from said formation.

2. The method as in claim 1 in which pumping is carried out substantially continuously during said period of at least 30 days.

3. The method as in claim '1 in which the injection of heated mining fluid is carried out at a pressure at least percent higher than the normal overburden lifting pressure of the formation penetrated substantially without destroying the self-containment of the dynamic, expanding, high-pressure zone.

4. The method as in claim 1 in which the injection of heated mining fluid is carried out at a pressure at least 100 percent higher than the normal overburden lifting pressure of the formation penetrated substantially without destroying the self-containing of the dynamic, expanding, high-pressure zone.

5. The method as in claim 1 in which the formation penetrated is a shale formation having an average grain size of 0.004 mm. diameter and an average permeability of 0.1 millidarcy to air.

'6. The method as in claim 1 in which the formation has been previously fractured.

7. The method as in claim 1 wherein the underground formation is a shale formation containing a substantial quantity of hydrocarbon material.

8. The method as in claim 1 in which at least 120,- 000 gallons of heated mining fluid are employed.

9. The method as in claim 1 in which the mining fluid employed is selected from the group consisting of Water, mineral oil, aqueous mineral acid, ethylene glycol, propylene glycol and brine.

10. The method as in claim 1 in which the mining fluid is injected into the formation at a wellhead pressure of at least 2,000 pounds per square inch.

11. The method as in claim 1 in which the mining fluid is injected into the formation at a wellhead pressure of at least 3,000 pounds per square inch.

12. The method as in claim 1 in which the mining fluid injected into the formation has a wellhead temperature of at least 250 C.

13. The method as in claim 1 in which said borehole is one of a plurality of boreholes and the mineral values are recovered from at least some of the balance of the boreholes which thus serve as recovery wells.

14. The method of mining a substantially impermeable underground earth formation at least 1,000 feet deep with a mining fluid capable of separating mineral values from the country rock in said formation which comprises: providing a plurality of spaced-apart boreholes each of which penetrates said formation, at least one of said boreholes serving as an injection well and the balance of the boreholes serving as recovery wells; over a period of at least 30 days, pumping down the at least one borehole and injecting into the formation a total of at least 20,000 gallons of heated mining fluid with no more than temporary stoppages in pumping and While substantially maintaining the pressure against the formation whenever pumping is temporarily stopped; said heated mining fluid having at the wellhead a temperature at least C. above the ambient temperature of the underground earth formation;

the heated mining fluid in the earth formation being at a sufliciently elevated temperature that a dynamic, expanding, self-contained, high-pressure zone is created and maintained;

the at least one injection well being sufliciently spaced apart from the at least one recovery well that the self-contained zone does not spread from the at least one injection well to the at least one recovery well in less than one months time;

and recovering mining fluid containing mineral values from the at least one recovery well.

15. The method as in claim 14 in which injection is continued over a period of at least six months and recovery is carried out over a period substantially greater than the injection period.

16. The method as in claim 14 in which the formation is a clay-type shale containing hydrocarbon material and the mining fluid has a temperature of from about 275 to 395 degrees centigrade at the wellhead, whereby hydrocarbon values in the formation are retorted by the mining fluid.

17. The method as in claim 16 in which a combustion supporting gas selected from the group consisting of air and oxygen is injected into the high-pressure zone after retorting has commenced, and igniting carbon or metallic sulfide containing material and carrying out a preselected extent of combustion thereby to further heat the formation within the high-pressure zone.

18. The method as in claim '14 in which the formation contains metallic sulfide and sulfur values are recoverable therefrom in a commercially attractive amount.

19. The method as in claim 18 in which a combustion supporting gas selected from the group consisting of air and oxygen is injected into the high-pressure zone after the formation has been heated to a temperature of at least 550 to 700 degrees centigrade, and igniting the metallic sulfide and carrying out a preselected amount of combustion thereby to generate recoverable oxides of sulfur and to further heat the formation Within the high-pressure zone.

References Cited UNITED STATES PATENTS IAN A. CALVERT, Primary Examiner U.S. Cl. X.R. 

